Wireless base stations such as mobile phone networks amplify and send CDMA signals and OFDM signals. The differences between the average values and the peak values of the transmission powers of these signals roughly range from 7 dB to 10 dB. A normal high-frequency power amplifier faces a problem that the conversion efficiency from power source to high-frequency power for the peak output power is high whereas the conversion efficiency for the average output power is low and the power consumption increases. Therefore, various types of high-frequency power amplifiers have been developed including Doherty type amplifiers and Out-Phasing type amplifiers, with which the conversion efficiency for the average output power can be high and the power consumption can be decreased.
In a general wireless base station, a signal processing unit processes transmitting signals to suppress the peak powers and distorts the signals and then the transmitting signals are input into an amplifier.
FIG. 1 is a diagram illustrating a configuration example of a Doherty type amplifier. A signal processing unit in the Doherty type amplifier processes transmitting signals and the processed signals are subject to a frequency conversion and converted into a frequency used for transmission. The converted signals are branched into a carrier amplifier and a peak amplifier. The carrier amplifier is generally biased from Class B to Class AB so as to start the operation from an area in which the input signal power is low. On the other hand, the peak amplifier is generally biased to Class C so that the peak amplifier does not operate in the area in which the input signal power is low and the amplifying element is switched off so that the power consumption is kept low. Thus, the carrier amplifier operates and the peak amplifier does not operate until the input signal power reaches a certain level. And the conversion efficiency of the amplifier maximizes at the level. After the input signal power exceeds the level, the peak amplifier starts to operate and the output power increases. And the conversion efficiency decreases once and then maximizes again at the level in which both amplifiers reaches a saturation range. In the Doherty type amplifier, a λ/4 line is inserted between the carrier amplifier and the output coupled point. The λ/4 line converts the output load impedance to impedance near the high-efficiency operation point of the carrier amplifier when the peak amplifier is switched off. It is noted that the λ generally means a wave length at a center frequency.
FIG. 2 is a diagram illustrating a configuration example of the Out-Phasing type amplifier. A signal processing unit in the Out-Phasing type amplifier processes transmitting signals and the processed signals are separated into two signals with different phases and certain amplitude by an amplitude phase conversion unit. The processed signal is separated into two reversed-phase signals when the amplitude of the processed signal is zero and into two in-phase signals when the amplitude of the processed signal maximizes. The two separated signals are subject to a frequency conversion and converted into signals with frequencies used for transmission and input into amplifying elements. Since signals with the fixed amplitude are input into the amplifying elements, the amplifying elements can operate with high conversion efficiency. Each output from the amplifying elements is subject to a vector synthesis in a synthesizer. As a result, the synthesizer outputs the input signals as amplified transmitting signals. The synthesizer generally includes the first transmission line and the second transmission line with the total length of the half wavelength. The efficiency of the Out-Phasing type amplifier can be high even at the output power level below the peak output power.